1. Field of the Invention
The present invention relates to a fixing device used in image forming apparatuses such as an electrophotographic copying machine and an electrophotographic printer.
2. Description of the Related Art
An external heating type fixing device is known as a fixing device used in an electrophotographic copying machine and an electrophotographic printer. Generally, this fixing device includes a fixing roller, a pressure member for forming a fixing nip portion cooperatively with the fixing roller, and a rotatable heating member for forming a heating contact portion cooperatively with the fixing roller. A rotatable endless belt which moves while making contact with a ceramic heater or a rotatable heating roller, in which a halogen heater is built therein, is used as the rotatable heating member. A recording material on which an unfixed toner image is borne is heated while being conveyed by the fixing nip portion, so that the toner image is fixed to the recording material.
In the external heating type fixing device, in a case where a jam occurs while the fixing nip portion conveys the recording material on which the unfixed toner image is borne, the toner may stick to a surface of the fixing roller. The toner adhering to the surface of the fixing roller may also stick to the surface of the rotatable heating member at the heating contact portion with the rotation of the fixing roller. After the toner adhering to the surface of the rotatable heating member is accumulated on the surface of the rotatable heating member and becomes a large lump, this lump toner may randomly fall on the recording material or return to the surface of the fixing roller, so as to smear the recording material to an extent that the image is regarded as a defective image.
Therefore, Japanese Patent Application Laid-Open No. 2003-114583 discusses a fixing device capable of causing release properties of the heating member with respect to the toner to be higher than release properties of the fixing roller. In the fixing device, because adhesion of the toner and the fixing roller is greater than adhesion of the toner and the heating member, the toner on the fixing roller is less likely to stick to the heating member and remains on the surface of the fixing roller. Therefore, the toner on the surface of the fixing roller can be gradually fixed to the recording material and discharged along with the rotation of the fixing roller, so that the recording material is less likely to be smeared to the extent that the image thereon is regarded as a defective image.
However, with the above-described external heating type fixing device, in a case where a recording paper is used as a recording material, a paper dust such as a paper fiber included in the recording paper may stick to the surface of the fixing roller at the fixing nip portion. The paper dust adhering to the surface of the fixing roller and the toner adhering to the surface of the fixing roller stick to the surface of the rotatable heating member to mix with each other, so as to form a sticking substance having a characteristic different from that of the toner. Therefore, the configurations discussed in Japanese Patent Application Laid-Open No. 2003-114583 may not be sufficient.